In an internal combustion engine, although a part of fuel that is injected into an intake port from a fuel injector vaporizes in the state it is in when it is injected, the remainder adheres temporarily to a wall surface (including an intake valve; the same applies hereunder) of the intake port. The fuel that adheres to the intake port is evaporated by a negative pressure inside an intake pipe or the action of heat from the intake port wall surface, and forms an air-fuel mixture together with a vaporized part of fuel that has been newly injected from the fuel injector. At a time of steady operation, there is a balance between the amount of fuel that is injected from the fuel injector and adheres to the intake port, and the amount of fuel that has been adhered to the intake port that vaporizes. Therefore, by injecting a fuel amount that corresponds to the theoretical air-fuel ratio from the fuel injector, it is possible to make the air-fuel ratio of an air-fuel mixture that is formed in a cylinder equal to the theoretical air-fuel ratio.
However, when starting an internal combustion engine, particularly at cold start-up, the temperature inside the intake pipe and the temperature of the intake port wall surface are low, and furthermore, a negative pressure is not yet generated inside the intake pipe. Further, the amount of fuel that is adhered to the intake port from prior to start-up is not large. Therefore, a large portion of the fuel that is injected from the fuel injector at start-up adheres to the intake port. Hence, in order to form an air-fuel mixture of an ignitable concentration inside a cylinder, in at least the initial cycle when starting the engine, it is necessary to supply a large amount of fuel in comparison to a time of steady operation after warming up is completed. Further, since fuel supply is performed in cylinder units, in the case of a multi-cylinder internal combustion engine that has a large number of cylinders, a large quantity of fuel is supplied in sequence to each cylinder. However, when a large quantity of fuel is supplied, a proportionately large amount of unburned hydrocarbon (HC) is discharged to an exhaust passage from inside the respective cylinders. Although a catalyst for purifying exhaust gas is disposed in the exhaust passage, because the temperature of the catalyst is low at start-up, a certain period of time is required until the purification ability of the catalyst is activated. Accordingly, it is desirable to suppress the discharge of unburned HC as much as possible from inside the cylinders at least until the catalyst is activated. Reducing unburned HC that is generated at start-up is ranked as one of the important issues for motor vehicles that have an internal combustion engine as a motive force.
Various kinds of technology have been proposed to solve the above problem. Among these, Patent Literature 1 that is mentioned below (hereunder, referred to as “prior art”) discloses technology that relates to the supply of fuel when starting a multi-cylinder internal combustion engine. As is also described in Patent Literature 1, it is not always necessary to supply fuel to all cylinders in order to start-up a multi-cylinder internal combustion engine, and it is possible to start the internal combustion engine even if the fuel supply to some of the cylinders is stopped. By starting up an internal combustion engine in a manner in which the fuel supply to some of the cylinders is stopped, it is possible to significantly reduce the amount of unburned HC that is discharged at start-up. The aforementioned prior art is an invention that is based on such knowledge, and is configured so as to determine which cylinders to supply fuel to and which cylinders to stop the supply of fuel to based on the result of a cylinder determination that is performed at start-up, and to control the fuel supply to each cylinder in accordance with the determination result. More specifically, according to the aforementioned prior art, a pattern for supplying fuel among cylinders is determined according to the water temperature at start-up. A plurality of fuel supply patterns that depend on whether the water temperature is high or low are prepared. The patterns are set so that a pattern that corresponds to a high water temperature stops the fuel supply to a large number of cylinders, while a pattern that corresponds to a low water temperature stops the fuel supply to a small number of cylinders. After start-up is completed (when the engine speed exceeds 400 rpm), fuel supply is performed to all of the cylinders.